This invention relates in general to multi-frequency tone generators, and more specifically to a digital tone generator capable of generating digital multi-frequency tones, including digital dual tone multi-frequency tones (DTMF) which are readily converted into analog tone signals.
Historically, oscillators that could be used for frequency generation were based on resonant circuits that used inductors and capacitors. These types of oscillators were used in early DTMF circuits, but to get frequencies that were stable enough and that did not drift, the inductors and capacitors needed to be expensive high grade parts. Advances in crystal oscillator technology made the oscillators more stable and less costly. Crystal oscillators and simple frequency dividers became more common as frequency generation circuits. Such circuits became the basis for tone generation required to generate alert tones in many electronic devices, such as selective call receivers and pagers.
DTMF signaling, as commonly used in electronic tone dialing, represents a telephone number encoded as a combination of high and low band frequencies corresponding to each digit of the telephone number. For example, the digit 2 is represented by combining a low band frequency of 770 Hz with a high band frequency of 1336 Hz. Each telephone number digit will therefore have a unique combination of a low band frequency and a high band frequency, to thereby form the DTMF signal.
The DTMF signal must have low distortion in order for the Central Telephone Office to determine the proper telephone number being dialed. Increased distortion of the DTMF signal is more likely to occur if the signal is transmitted from a transducer into the telephone microphone instead of by direct electrical connection to the phone line.
To generate DTMF signaling, two analog sine waves are required. Due to the frequency accuracy required, this is usually done by starting with a high frequency crystal oscillator, such as having a typical operating frequency of 3.5 MHz, as shown in FIG. 1. The high frequency oscillator is divided down by a clock divider and fed to an index counter. The index counter generates a repeating modulo count which is connected to a read only memory (ROM) which feeds a digital to analog (D to A) converter. The ROM contains a sine wave table, so as the modulo count generated by the index counter is incremented, the D to A converter produced a sine wave output. The frequency of the sine wave would be adjusted by changing the divide ratio of the clock divider between the high frequency crystal oscillator and the index counter. The clock divider is changed by loading the divide ratio directly, or by way of a frequency decoder which generates the desired divide ratio from a frequency selection input. Two clock dividers with different divide ratios, two index counters, and two D to A converters were required to generate two sine waves at different frequencies. Once the two analog sine waves have been generated, they were combined together. The combined signal was then coupled to an analog filter to smooth out any transitions caused by the digital generation of the sine waves. The output of the analog filter was delivered to a analog driver circuit, and then to either a phone line or a speaker. A commercially available integrated circuit which operates as described above is manufactured by Signetics Corporation of Sunnyvale, Calif., the Signetics PCD3311. The D to A converters and low pass filters which were used in prior art integrated circuit technology generally required complex circuits which typically increased die size and cost.
A less common method for producing DTMF signaling was used with limited success in selective call receivers and pagers. This less common method generated two square wave signals at low and high frequencies corresponding to the DTMF tones desired using two digital timer circuits commonly available on commercial microprocessors, as shown in FIG. 2. The frequencies of the square wave signals were set by loading values into the MCU counter/timers over the data bus. The MCU counter/timers were clocked at the frequency of the MCU oscillator, and the resultant square wave signals generated were then filtered using analog filter circuitry to generate the desired low and high frequency DTMF tones, which were then added together with another analog circuit. Once added together, the combined signal was fed to an analog driver circuit. Analog filtering that could transform the square waves into sine waves of low enough distortion for use as DTMF tones were relatively expensive and difficult to design, considering the low voltages available within a pager. The analog driver circuit was also typically expensive and increased power requirements within the pager.
Pulse-width modulation technology has been used for volume control of alerts, such as in pagers. Prior art technology chopped the audio alert signal with a high frequency signal to control the volume level of the alert. The prior art technology used the modulation only to adjust the volume level of the alert, and the modulation was not used to create or vary the waveform of the audio signal. The pulse-width of the modulation was generally fixed. While this method of pulse-width modulation is considered adequate for many operational conditions, it is not without need of improvement for DTMF application.
Fractional dividers were also introduced into pagers to generate audio alert tones (less than 4 KHz) from a low frequency oscillator, typically 32 KHz. The tones generated by the fractional dividers were generally not suitable for use in DTMF circuits due to the jitter and distortion inherent in their generation.
Music synthesizers were originally designed using a series of inductor/capacitor oscillators that could be mixed with each other using analog circuitry to generate various frequency waveforms with various attack and decay envelopes. The complexity and cost of these analog circuits have always been too expensive to implement in low cost products, such as pagers. The advances in crystal oscillators and digital technology have allowed increasingly more complex circuits to be implemented in low cost products. However, the analog waveforms derived by conventional tone generating circuits were not easily changed to control the attack and decay envelopes necessary for music synthesizers, and can not be directly transferred to digital circuitry without maintaining some expensive digital to analog converter and driver circuitry.
Thus what is needed is an apparatus for use in electronic devices for generating tones, especially those electronic devices which operate from a single cell battery.
What is also needed is an apparatus for use in electronic devices for generating tones, especially for generating tones which are used to provide an audible alert.
What is also needed is an apparatus for use in electronic devices for generating tones, especially for generating tones which are used to provide an audible melody alert.
What is also needed is an apparatus for use in electronic devices for generating an all digital waveform that can readily be transformed into an analog signal.
What is also needed is an apparatus for use in electronic devices that will generate an all digital waveform and that will function and sound essentially the same as the more expensive analog circuitry.
What is also needed is an apparatus for use in electronic devices that can simply and effectively control the envelope of a digital tone generator to provide the tonal qualities of a musical instrument.
What is also needed is an apparatus for use in electronic devices for generating tones, especially for generating DTMF tones which are used to acoustically dial a telephone number.
The above and other features and advantages of the present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings.
In accordance with one aspect of the present invention, a digital tone generator comprises a digital signal generator which generates a digital waveform data sequence which is representative of a tone having a predetermined waveform, and a pulse-width modulator which is responsive to the digital waveform data sequence for generating a pulse-width modulated digital tone signal.
A further aspect of the present invention is the digital tone generator which further comprises a low pass filter which is responsive to the pulse-width modulated digital tone signal for generating an analog tone signal.
In accordance with another aspect of the present invention, a digital DTMF tone generator comprises a first digital signal generator which generates a first digital waveform data sequence which is representative of a high DTMF tone having a predetermined waveform, and a second digital signal generator which generates a second digital waveform data sequence which is representative of a low DTMF tone having a predetermined waveform. A digital adder adds the first digital waveform data sequence and the second digital waveform data sequence to derive composite DTMF tone data. A pulse-width modulator next encodes the composite DTMF tone data sequence to generate pulse-width modulated DTMF tone data.
A further aspect of the present invention is the digital DTMF tone generator which comprises a low pass filter which is responsive to the pulse-width modulated DTMF tone data for generating an analog DTMF tone signal.
A still further aspect of the present invention is an adaptive pulse-width modulator for pulse-width modulating a digital tone signal. The adaptive pulse width modulator comprises an index table which stores a table of on-state counter values and off-state counter values which are selected in accordance with the digital tone signal. A selection circuit is responsive to a mode selection signal for alternately selecting an on-state counter value and off-state counter value selected from the table of on-state counter values and off-state counter values by the digital tone signal. A counter is responsive to a reference clock for providing a count representing the on-state counter value and off-state counter value selected, and in response thereto generates an end-of-count signal. A mode selector is responsive to the end-of-count signal for generating the mode selection signal, and the mode selector also delivers a pulse-width modulated digital tone signal.